1. Field of the Invention
An object of the present invention a device to detect the depassivation of an integrated circuit. In semiconductor technology, it relates more particularly to the field of electronic credit cards where electronic integrated circuits are used, to keep the accounts of individual bank balances up to date and/or, also, as keys providing access, through a recorded secret code, to bank machines (for example automatic money tills).
2. Description of the Prior Art
One of the biggest problems encountered with respect to integrated circuits incorporated in electronic credit cards relates to the risk of fraud. In view of the confidence placed in this new means of financial transaction, the inviolability of these cards should be ensured under all circumstances. Thus, for example, systems have been invented to prevent the use of the d.c. power supply circuit of an electronic credit card incorporating integrated circuits of this type to try and discover the secret codes which such a card may contain. To this end, the voltage, usable inside the integrated circuit in order to bias it, has been made independent of the supply voltage. Furthermore, the output circuits of these integrated circuits have been modified so that this power supply does not put through different currents depending on whether the logic states read in the memory of the integrated circuit and transmitted are logic states "1" or logic states "0". Following another line of thinking, the architecture of the contact terminals which can be used for the programming of these integrated circuits has been modified so that, after this programming, any action taken at these terminals is ineffective. However, the imagination of fraudulent persons knows no bounds.
In fact, it has been realized that the operation of an integrated circuit can be known and understood when the passivation layer that covers this circuit after its manufacture has been removed, and when memory points of its memory are selected while, at the same time, observing the passage of the current through the connections of this integrated circuit with an electron microscope. Although this method of fraud is very expensive, protection against it is becoming a necessity.
It is an object of the invention to cope with this particular problem of fraud. To resolve this problem, the invention uses the fact that the capacitance displayed by a line, a connection of this integrated circuit, with respect to another line of this integrated circuit or with respect to a general ground is modified by the removal of this passivation layer. For this line capacitance is far greater when the passivation exists and, on the contrary, it is weaker when this passivation has been removed. On the basis of this observation, in the invention, a detector comprising a critical triggering threshold is connected to the terminals of a line with a capacitance of this type, this critical threshold being set in between the electrical states which the considered line is liable to take before and after the depassivating operation. It is possible to use lines specifically made for this purpose. It is also possible to use lines already existing in the integrated circuit block, lines of the memory block for example. Besides, since the depassivation detector may be very small (in one example it has six transistors), it is also possible to place a number of these detectors at several places on the integrated circuit so as to prevent these detectors from being neutralized by an electrode brought in from outside. If there are too many of these transistors, the number of neutralizing electrodes will be too great and will mask the field of the electron microscope.
The invention therefore concerns a device to detect the depassivation of an integrated circuit, comprising means to detect a variation in a line capacitance resulting from this depassivation.